The invention relates to a method for spraying workpieces with a fluid, and, such as a lubricant, and an associated device for spraying workpieces with a fluid, such as a lubricant.
A method of this type is described in DE 195 44 016 A1.
In this specification, the adjustment of the quantity of operating fluid atomised by the spraying head is effected over a limited range by means of a motor-adjustable nozzle needle.
By way of the present invention, a method of the initially stated type is to be provided, which allows for the adjustment of the quantity per unit of time of operating fluid atomised by the spraying head over a wider range.
This object is attained according to the invention by the features disclosed in claim 1.
In a method according to the invention, use is made of the fact that the operating fluids which are usually to be applied to workpieces are viscous. In the temperature range of interest in this case (room temperature to 80xc2x0 C.), the viscosity of fluids typically varies approximately proportional to the square of the ratio (in C.xc2x0) between the difference between the temperature T (xc2x0 C.) in question and the melting temperature of the water Ts and room temperature Tr (xc2x0 C.) and Ts, i.e. proportional to (Txe2x88x92Ts)2/(Trxe2x88x92Ts)2. At T=80xc2x0 C., the viscosity is therefore 16 times lower than at room temperature.
In practice, application quantities of approximately 0.8 to 10 g/m2 are obtained.
The rate of operating fluid passing through the spraying head can also be adjusted via the pressure at which the operating fluid is supplied to the spraying head. In practice, it is possible to attain a throughput control by a factor of approximately 2 by varying the supply pressure between 5 bar and 10 bar.
If both possibilities of rate control are combined, it is possible to obtain an overall adjustment range of the throughput by a factor of approximately 25. The temperature adjustment and the pressure adjustment can be carried out with precision by using conventional control methods, so that precise application quantities can also be ensured. Now the application quantity and temperature or pressure relate to one another in detail depends upon the physical (and to some degree also the chemical) properties of the respective operating fluid and the respective spraying heads which are used. The method according to the invention is preferably used in such a manner that these dependencies are firstly determined in tests, the test results are deposited in stores and by using these stored test results the temperature and/or pressure is/are adjusted in such a manner that the desired application quantity of operating fluid is obtained.
Typical operating fluids are, more particularly, oils having various physical and chemical properties, as are used for spraying metal sheets prior to pressing or deep drawing operations.
Advantageous developments of the invention are contained in subclaims.
The development of the invention in a preferred embodiment allows for careful heating of the operating fluid by using a heating medium having a higher temperature over a large contact area. In contrast, the introduction of corresponding quantities of heat directly from an electrical heating device could result in losses in quality of the operating fluid.
One embodiment of the invention offers the advantage that a large quantity of heat can be supplied in a compact volume. The use of water as a heating fluid also offers the advantage that it is possible to fall back on the tried and tested technology of water heating devices and water heat exchangers. Furthermore, water also has relatively good thermal conductivity.
The development of the invention in one embodiment allows for a simple continuous and reproducible supply of heat to the heating fluid.
The development of the invention in another embodiment allows for a further increase in the throughput adjustment range.
By way of one embodiment of the invention, an overall fine adjustment of the application quantity is obtained, and it is also possible to ensure a short control constant as compared with a temperature adjustment alone by dividing the overall adjustment between a temperature adjustment, a pressure adjustment and a key time adjustment.
For some applications, it is advantageous if the opposing sides of a workpiece (e.g. metal sheet) can be differently sprayed with operating fluid. In this manner, it is possible to take into account that the mechanical stresses to which both surfaces are subjected during the subsequent treatment of the workpieces (e.g. deep drawing of a metal sheet), can differ. It is also possible, if desired, to spray the two sides of the workpiece with different operating fluid.
The development of the invention as claimed in claim 8 also allows for a local variation in the application quantity when spraying only a single side according to the respective anticipated requirements. Thus, for example, it is possible to coat areas of metal sheets which are to be pressed which undergoes minimal deformation in the pressing mould with a small quantity of operating fluid, whilst spraying areas which are heavily deformed in the pressing mould with a large quantity of operating fluid.
With the present invention, the temperature adjustment is effected in a connecting line leading to the spraying heads, which in practice is a ring conduit. The heating device provided to this end therefore only needs to have a smaller heating capacity than in cases where the temperature of the entire supply of operating fluid in a supply receptacle is adjusted. A further advantage of the solution according to the invention is that the temperature adjustment takes place with a smaller control constant, since the respective volumes which need to be adjusted to the desired temperature are small as compared with the content of a supply receptacle.
The development of the invention in one embodiment is again advantageous with a view towards careful heating of the operating fluid.
In one embodiment of the device, a circulated heating medium is used, in which only the arising heat losses are compensated. This is advantageous with a view towards low energy consumption and with a view towards corrosion protection of the primary circuit of the heat exchanger.
The developments of the invention in certain embodiments are advantageous with a view towards a rapid and precise adjustment of the temperature conditions in the primary circuit and in the secondary circuit of the heat exchanger. Thus, in cases where a temperature increase is desired, it is possible to firstly supply considerably more heat to the primary circuit than is required during stationary operation. As a result of the constant monitoring of the temperatures, overshooting is prevented as well as an excessively slow approach of the temperature to the stationary end value.
The development of the invention in certain embodiments ensure monitoring of the fluid flows in the primary and secondary circuits of the heat exchanger. From the output signal of the flow meter or flow sensor, it is also easily possible to determine if a circulating pump fails or if other disturbances occur during the conveyance of the fluids.
In certain embodiments, a heat exchanger can be manufactured in a very simple manner from finished components which only need to be brought into contact with one another with sufficiently large surface areas.
In one embodiment, the duct of the heat exchanger conveying the operating fluid can simultaneously undertake a supportive function for the spraying heads as well as a distributive function for the latter.
In another embodiment, the entire heat exchanger can be manufactured very simply and economically as an integrally formed part.
The development of the invention in another embodiment is advantageous with a view towards a constant temperature adjustment in the longitudinal direction of the heat exchanger.
The development of the invention in another embodiment is advantageous with a view towards uniform heating of the operating fluid from both sides.
In the device in one embodiment, compressed air, which a spray head possibly required for atomisation or for shaping the generated mist, is heated to the temperature of the operating fluid before it is supplied to the spraying heads.
In this respect, the development of the invention in one such embodiment is again advantageous with a view towards a simple realisation both of the first heat exchanger and of the second heat exchanger by a single extruded multi-chamber profile.
The development of the invention in another embodiment allows for a simple, clear fitting, which is also expedient for maintenance, of the spraying heads to a duct conveying the operating fluid.
In another embodiment, it is possible in a simple manner to maintain the spraying heads per se at the same temperature as the supplied operating fluid.
In this respect, the development of the invention in one such embodiments is advantageous with a view towards a uniform temperature adjustment in the spraying head.
In one embodiment of the device, the compressed air supply of the spraying head is also effected via the adapter element supporting said spraying head. No separate compressed air lines are therefore required. The spraying head can be dismantled and fitted in a particularly simple manner.
In one embodiment of the device, the compressed air control associated with a spraying head is spatially combined in a simple manner with the adapter element, which is in turn advantageous for reasons relating to assembly and facility of inspection.
The development of the invention according to one embodiment means that it is possible to carry out a final fine adjustment of the temperature of the fluid which is to be atomised by the spraying head directly at the spraying head. In this manner, it is possible to account for locally varying ambient temperatures in the case of long spraying head strips. Thus, for example, the spraying heads lying in the centre of a spraying head strip follow the temperature of the adjacent spraying heads, so that their thermal losses are less than those of the peripheral spraying heads. The different spraying heads of a spraying head strip can also lie in different flows of the surrounding atmosphere, partially as a result of the design of a protective housing enclosing the spraying heads or the spraying head strips, partially as a result of the air flows at the installation site (factory hall).
In this respect, the development of the invention in one such embodiment is advantageous with a view towards a simple automatic temperature adjustment at the spraying heads.
According to another embodiment of the invention, the electrical cabling of a spraying head strip can also be fitted in a compact manner allowing for ease of inspection.
The developments of the invention in one preferred embodiment are advantageous with a view towards simple replaceability of the individual spraying heads and the adapter element associated therewith, without major intervention into the overall installation.